Magnetic field sensors are used in a variety of applications, including, but not limited to, an angle sensor that senses an angle of a direction of a magnetic field, a current sensor that senses a magnetic field generated by a current carried by a current-carrying conductor, a magnetic switch that senses the proximity of a ferromagnetic object, a rotation detector that senses passing ferromagnetic articles, for example, magnetic domains of a ring magnet or a ferromagnetic target (e.g., gear teeth) where the magnetic field sensor is used in combination with a back-biased or other magnet, and a magnetic field sensor that senses a magnetic field density of a magnetic field.
In certain applications, magnetic field sensors include magnetoresistance elements. These elements have an electrical resistance that changes in the presence of an external magnetic field. Spin valves are a type of magnetoresistance element formed from two or more magnetic materials or layers. The simplest form of a spin valve has a reference (or magnetically fixed) layer and a free layer. The resistance of the spin valve changes as a function of the magnetic alignment of the reference and free layers. Typically, the magnetic alignment of the reference layer does not change, while the magnetic alignment of the free layer moves in response to external magnetic fields.
In some cases, a spin valve may also have a bias. The bias may be generated by one or more magnetic layers (bias layers) that are magnetically coupled to the free layer. In the absence of an external magnetic field, the bias layers may cause the magnetic alignment of the free layer to default to a predetermined alignment. The magnetic coupling between the bias layers and the free layer is relatively weak so that an external field can override the bias and realign the magnetic alignment of the free layer.